Buffett tapped Kass to serve as one of his three interrogators on Saturday, an event widely known as "Buffettstock" in Omaha, Neb. But Kass' negativity about Berkshire didn't make Buffett flinch.

Indeed, Buffett's number two, Charlie Munger, chided Kass, saying he could "make the short-seller's argument even better than he did."

Kass said that Berkshire was in danger of becoming an index fund due to its sheer size, forcing it to hunt "elephants" rather than "gazelles." He also said he fears that the 82-year-old Buffett's successors won't enjoy the same advantages.

To the first criticism, Buffett acknowledged that Berkshire will not grow as much as it has in the past, but that it will still generate value. To the second, he said Berkshire's brand will supplant his own when he finally retires.